Since the early 1970s, heavy gauge thermoplastic sheetstock has been used to produce custom orthoses (orthopedic braces or appliances) and prostheses (artificial limbs) and components thereof, usually by way of a vacuum forming process. The methodology of clinical vacuum forming with thermoplastic sheetstock was first explored at numerous medical centers around the United States (Vacuum Forming of Plastics in Prosthetics and Orthotics, A. Bennett Wilson Jr., Orthotics and Prosthetics, Vol. 28, No 1, pp. 12-20, March 1974). The advent of “total contact” orthoses offered an alternative to historical leather and metal fabrication of conventional orthoses (Thermoformed Ankle-Foot Orthoses, Stills. M, Orthotics & Prosthetics, Vol. 29, No 4, pp. 41-51, December 1975).
Clinical products made in this fashion offer good durability, hypoallergenic patient contact surfaces and the ability to mold a product that is very intimate with the body segment of the patient that requires external support alignment or stabilization.
A custom orthoses (orthosis) or (prosthesis) prosthetic device (alternatively orthotic or prosthesis device, orthotic device, prosthetic device, clinical product, custom clinical product or in the plural orthoses and prostheses) is often fabricated by first making mold of the portion of the body over which the device is intended to be worn. Current practice is to cast the body part with plaster of Paris bandages or synthetic casting tapes. Once cured and removed from the body, the plaster or synthetic cast is called a negative model. To turn this into a positive model, the negative model is then dammed off and filled with a slurry of plaster of Paris, which is then allowed to harden. The negative cast is then removed from the surface of the positive model, and a plaster rendition of the body segment is provided as an unimproved positive model. The positive model is then improved for various clinical implications offering correction, alignment, stabilization and protection. The improved positive model is then used as a mold to fabricate a custom clinical product, specifically designed to fit that particular patient's body part.
The human form is a variable commodity, and changes in volume and shape with respect to growth, weight gain and underlying clinical patho-etiologies and pathomechanics which frequently occur. As a result, orthotic or prosthetic devices frequently need to be adjusted in the post delivery and patient use stage. This is usually done by a practitioner orthotist or prosthetist (i.e. an orthosis or prosthesis device expert) as the patient's body undergoes change.
In order to meet this need for easy adjustment, the custom clinical product is often fabricated using thermoplastic sheetstock. This material has certain advantages for forming orthotic or prosthetic devices because its shape can be easily adjusted by heat molding (viscoelastic remodeling) the material in the post delivery and patient use stage.
To make these adjustments, a practitioner will often make spot or isolated changes in the device through the use of a heat gun. A heat gun produces a directed stream of heated air. The delivery of the heated air can be directed to a spot in the clinical device that may be causing discomfort to the patient due to pressure and laminar sheer against the skin and underlying skeletal prominence. The heated spot can be re-contoured through viscoelastic remodeling of the thermoplastic sheetstock, with no reduction in the strength of the cooled thermoplastic material.
Orthotic fabrication is typically done under very low pressures and at temperatures that are roughly around the melt temperature of the thermoplastic material. Here, for example, thermoplastic sheetstock that has been heated to roughly melt temperature may simply be grasped by hand (using insulated gloves) at the four corners of the sheet, draped over the plaster of Paris positive mold of the body part, and deformed or reformed to fit the mold by low pressure vacuum forming. To do this low pressure (open atmosphere) vacuum forming, a vacuum is applied to the internal space between the thermoplastic sheetstock and the positive model which removes any residual air in the captured space. The force of the ambient (atmospheric) air pressure then intimately molds the thermoplastic sheetstock to the surface of the positive model. This forming process is facilitated by the fact that thermoplastic sheetstock is generally self-adhesive at melt temperature, and will seal to itself during this process, eliminating the need for any accessory adhesive for the airtight seal around the positive model. See for example, U.S. Pat. No. 2,978,376 (Hulse).
Typical thermoplastic resins used for thermoforming may include ABS, Kydex®, Lexan®, VIVAK®, TPE, PVC, polystyrene and numerous other generic and proprietary resins. Thermopolymer polyolefin resins in the form of polyethylene (PE) were first developed in 1934 by ICI in the United Kingdom. Dupont opened the first PE plant to support the war effort in 1943. Polypropylene, another polyolefin variant, became suitable for heavy gauge thermoforming in the mid 1950s. Isotatic polypropylene is the most common type of polypropylene sheetstock that is used for vacuum thermoforming.
Prior art on the construction of orthosis or prosthesis devices includes U.S. Pat. No. 1,232,899 (De Puy), U.S. Pat. No. 3,916,886 (Rogers), and U.S. Pat. No. 4,289,122 (Mason & Vuletich).
Although pure homopolymer sheetstock thus has many advantages for these applications, this material also has a number of significant disadvantages.
One disadvantage of using a pure homopolymer sheetstock for cut sheet heavy gauge vacuum thermoforming is the relative lack of sheet strength at melt temperature. Sheetstock that is heated just a small degree over the recommended temperature molding range for the particular sag strength of the specific resin formulation can undergo a sharp change from high viscosity to low viscosity, and as a result droop very quickly in the transfer from the non-stick oven tray to the positive model, forming regions of non-uniform thickness. Sheetstock that is heated just a small degree over the recommended molding temperature can undergo a sharp change from high viscosity to low viscosity, and as a result droop very quickly, due to the poor sag strength, in the transfer from the non-stick oven tray to the positive model, forming regions of non-uniform and substandard thickness.
This sharp change in viscosity is a particular problem for sheetstock made from homopolymer polypropylene and copolymer polypropylene. Overly hot thermoplastic sheetstock can rapidly stretch during the hand-held manipulation process. This can form thin regions in the material, resulting in a final product that might not be stiff enough to be suitable as a clinical device, resulting in wasted materials and effort.
Another drawback of using pure homogenous thermoplastic sheetstock is that the final orthoses and prostheses made from such un-reinforced homogeneous thermoplastic sheetstock have a tendency to further deform or reform with use. That is, although the orthoses or prosthesis may originally fit the patient well, with use the devices will further deform or reform, and gradually fit the patient less well. This gradual deformation is an example of “creep” or, for a clinical orthosis or prosthesis device, this creep can be termed “clinical creep”. Thus an orthosis or prosthesis that might originally fit the patient well will, with use, end up fitting poorly.
Creep is a common characteristic of thermoplastic materials, especially semi-crystalline materials such as polypropylene and polyethylene and their variants. The polymer chains comprising the molecular structure of these resins are not chemically cross linked. As a result, the polymer chains will continue to move and allow the product to change shape over time, even when the use of the product is within the normal temperature service range for the particular resin. Creep is thus due to the natural viscoelastic properties of thermoplastic materials. Creep typically occurs in the amorphous area of the polymer chain structure and not within the crystalline area of a polyolefin resin.
Clinical creep or “creep” is thus a manifestation of the viscoelastic change that occurs in a clinical device fabricated from a thermoplastic material that will change shape due to the influence of gait forces that are applied to the device as well as the increase in temperature of the device from absorption of heat from contact with the human body. Clinical creep is a disadvantage in a lower extremity orthosis especially, because the foot and ankle complex requires the maintenance of optimal skeletal alignment, support and stabilization.
Even the patient's body heat can change the viscoelastic properties of thermoplastic sheetstock, and normal body temperature may raise the temperature of the device, which will accelerate clinical creep. The change in shape in the clinical product from clinical creep may decrease the efficiency of the device in the long term and lessen the useful life of the product. The polyolefin family of resins, which includes homopolymer polypropylene, co-polymer polypropylene and the polyethylene variants, are all very susceptible to clinical creep when used in a lower extremity orthosis.
Clinical creep is also a drawback in spinal orthoses that are utilized to stabilize or straighten the spinal column. The optimal corrective forces of a spinal orthosis will decrease as clinical creep alters that shape of the orthosis and will decrease the corrective effectiveness of the orthosis.
As in orthoses, the sockets of prostheses are susceptible to shape change due to clinical creep when fabricated using thermoplastic materials, especially polypropylene. For these sockets, such creep has made pure thermoplastic materials and fabrication virtually unsuitable for definitive, long-term use in prosthesis.
Resins are used to create many types of products in the modern world, and in areas outside of orthoses and prostheses, in order to combat creep and confer additional strength; it is common to impregnate various types of fibers into, or along with, the thermoplastic resin. These fibers resist stretching along whatever angle that the fiber is aligned, but are generally ineffective at resisting compression along whatever angle that the fiber is aligned, and they are also generally ineffective at resisting bending perpendicular to whatever angle that the fiber is aligned. For example, substrate reinforcing fibers can be impregnated with uncured or unpolymerized thermoset resin either before or after the reinforcing fibers are placed in a product mold. These reinforcing fibers can confer additional dimensional stability and robustness to the final product. This is the general principle behind fiberglass, for example, which is used for a wide variety of different applications.
Various ways to incorporate fibers into thermoplastic resins are known. These methods include U.S. Pat. No. 3,523,149 (Hartmann), US Statutory Invention Registration H1162 (Yamamoto et al.), U.S. Pat. No. 6,054,022 (Helwig et al.), U.S. Pat. No. 4,478,771 (Schreiber), U.S. Pat. No. 5,071,608 (Smith et al.), U.S. Pat. No. 5,194,462 (Hirasaka et al.), and U.S. Pat. No. 5,741,744 (Fitchmun).
In one method, powdered thermoplastic resin is introduced into a woven, braided or a textile form of the continuous substrate fibers. The resulting material (often called a “prepreg” because the continuous substrate textile is pre-impregnated with the resin by the manufacturer, and then often shipped to the end user in the form of ready to use sheets or rolls) retains the textile characteristic of being “drapable”. This dry powdered prepreg can then be molded under high pressure and heated in a matched two-sided mold to consolidate the fibers and resin into a finished product.
Thermoplastic prepreg sheets that contain continuous fiber reinforcement in woven or braided form are commercially available. However due to the fact that the fibers are continuous and are present in a woven or braided form, these sheets are generally quite stiff. In order to fit these stiff sheets into complex molds, typically both matched (i.e. upper and lower mold surfaces) and sturdy molds (often made of metal) and high pressure and heat are required.
Unfortunately, the molding pressures that are used in prior art thermoplastic prepreg sheets are too high for use on the temporary plaster of Paris positive models used in the fabrication of orthoses and prostheses. Thus at present, the state of the art in the orthoses and prosthetic device field is generally to form the main body of the orthoses and prostheses out of the pure homogenous thermoplastic sheetstock, and to reserve any fiber reinforcement for certain strategic regions of the orthoses and prosthesis where it is absolutely essential, as in U.S. Pat. No. 6,146,349 (Rothschild et al.) and U.S. Pat. No. 5,312,669 (Bedard).
Although there is prior art with regards to using reinforcing fibers for orthoses and prosthesis devices, such as U.S. Pat. No. 6,146,344 (Bader), U.S. Pat. No. 5,624,386 (Tailor), U.S. Pat. No. 5,693,007 (Townsend), the results are still not fully satisfactory, particularly when durable orthoses and prosthesis devices must be constructed that must conform to complex body shapes, such as a three dimensional shape with a double horizon bend. As a result, the problem of clinical creep persists, and present day orthoses and prosthesis have both a limited use lifetime and a need for continual readjustment.